1. Field of the Invention
The present invention relates to a method of defect detecting in an optical recording device, and in particular, to a method for preventing erroneous defect detection when the optical recording device switches from read to write.
2. Description of the Prior Art
The Mount Rainier disk recording format was developed by Compaq, Microsoft, Philips, Sony, and other companies, for defect management. For optical recording devices that utilize the defect management function, the general method of detecting defects is to take advantage of the fact that the sub-beam added (SBAD) signal generated by the optical head will be maintained at a certain level when the optical recording device performs writing to a disk. When a defect is encountered, the SBAD signal will vary due to the abnormal amount of the light reflected from the disk. In this way, the optical recording device is able to determine whether the disk contains a defect based on a function of level variations in the SBAD signal.
As shown in FIG. 1, in order to measure the level variations in the SBAD signal more accurately, one can first pass the SBAD signal through a low-pass filter to obtain a stable and slowly changing signal, which is called SBAD_Ipf signal. This SBAD_Ipf signal is used as the reference signal. During the writing process, the SBAD signal is compared with the reference signal (SBAD_Ipf signal). When the difference between two signals exceeds a preset upper limit (i.e., the SBAD signal is higher than the reference signal) or a lower limit (i.e., the SBAD signal is lower than the reference signal), a defect signal will be activated. The period of the activated defect signal corresponding to an area on the disk is determined as a defect area. The optical recording device will label the defect area according to the length and location of the defect, and then the data is written to a substitute location on the disk, pursuant to the Mount Rainier disk specification.
As shown in FIG. 2, if the threshold frequency of the low-pass filter for the reference signal (SBAD_Ipf signal) shown in FIG. 1 is set higher, the reference signal (SBAD_Ipf signal) will approach the SBAD signal more quickly and then cause the shorter period of the activated defect signal. This means that the defect area may not be detected completely. Therefore, when detecting whether a disk has defects, it is necessary to select an appropriate threshold frequency of the low-pass filter for the reference signal (SBAD_Ipf signal) to make the reference signal vary slowly so that the entire defect area can be detected.
Generally speaking, the SBAD signal will drop or rise suddenly at the instant that writing is started. For example, as shown in FIG. 3, when writing is started, the SBAD signal will suddenly drop to a level that is below the reference signal (SBAD_Ipf ), so the defect signal will be activated because a lower limit is detected. The period of the activated defect signal identifies the existence of a defect area. At that time period, however, the detected defect area is an erroneous defect detection. In fact, the power change of the laser from read to write also causes the SBAD signal to suddenly drop or rise even though there may be no defect area in the starting location of writing.
Consequently, when level variation of the SBAD signal is used to determine whether there are defect areas on a disk, it is very likely that the SBAD signal generated by the optical head will rise or drop suddenly due to the variation in the laser power of the optical head (as shown in FIG. 4) during the process when the optical head switches from read to write at the instant that the optical recording device initiates a write operation. In order to prevent erroneous defect detection, the gain of the SBAD signal can be adjusted to reduce the difference between the SBAD signal and the SBAD_Ipf signal before the time and at the time writing is started. However, this method cannot completely eliminate the aforementioned problem, because there still may be a difference that occurs that exceeds the upper limit or lower limit. As a result, the starting location for a writing operation may be misidentified or the actual defect may go undetected.
If there is a defect area on the disk and the optical recording device is unable to detect it during writing, the data stored at that location will be unreadable. If there is no defect at a particular location on the disk and the optical recording device erroneously identifies the existence of a defect, not only will storage space on the disk be wasted, but it will also take more time for the optical recording device to perform its read and write operations.
As shown in FIG. 5, in order to avoid defect detection failures that occur when the SBAD signal rises or drops suddenly at the beginning of writing, another conventional method of solving the problem is to reset the reference signal to have the same value with the SBAD at the instant that writing is started. However, the optical recording device might immediately encounter a defect just when writing starts (i.e., during the time that the optical head switches from reading to writing). In this case, the reference signal (SBAD—1pf) might quickly drop to the level of the SBAD signal at the starting point of writing. As a result, an erroneous defect detection occurs because of the slowly changing reference signal causing an appearance of an upper limit.
Thus, the above-described prior art methods for determining whether there are defective locations on a disk based on level variations of the SBAD signal and the reference signal still carry inconveniences and problems that need to be solved in practical application, since detection failures still tend to occur during the time that the optical head switches from reading to writing.